The present invention relates generally to battery separators for rechargeable alkaline cells and is more particularly concerned with a new and improved battery separator substrate well suited for use in alkaline batteries such as the nickel-zinc cell batteries.
In recent years the limits on available petro-chemical energy sources and the pollution problems attendent to internal combustion engines has engendered new interest in electrically powered automobiles. Heretofore one of the main disadvantages of such automobiles has been the lack of a practical rechargeable electrical energy source--namely, a battery capable of withstanding repeated deep discharge and peak performance after repeated recharging. The battery that appears to offer most promise in connection with this application is the rechargeable alkaline nickel-zinc battery using improved aerospace battery plate separators. This battery has three times the energy output of lead-acid or nickel-cadminum batteries of the same weight and can be produced at a comparable or lower cost. Additionally, the nickel-zinc type battery demonstrates an improved accelleration rate, can nearly double the driving range between recharging and can handle twice as many stop and go driving cycles per charge as comparable lead-acid batteries.
In attempting to optimize performance of the nickel-zinc battery it has been determined that one of the most important components of the battery is the separator and its ability to meet the cycle life requirements for electric automotive utilization. Additionally, it has been determined that separator cost is the most important priority in obtaining practical utilization of the nickel-zinc battery since the separator cost tends to control the total cost of the battery.
Heretofore research has centered on the inorganic/organic flexible separator developed by NASA. That separator comprises a nonwoven porous substrate such as an asbestos or cellulosic fiber base sheet covered or saturated with a coating mix of polyphenylene oxide, a polymeric polyester that reacts with the alkaline electrolyte and inorganic or organic fillers suited for forming the desired porous or micro-porous film. The asbestos substrate is of fuel cell grade and exhibits a thickness of about 10 mils. However, the health hazards associated with asbestos have received considerable publicity in recent years and its use is being phased out in many applications. The cellulosic fiber base sheet is merely newsprint and tends to degrade with time particularly within an alkaline environment. Also it is deficient in oxidation and abrasion resistance and tends to permit shorting of the battery due to zincate ion transfer and dendrite growth through the separator.
It has now been found that substantially improved separator substrates of extremely thin light weight web materials can be prepared at very low cost while avoiding the use of health hazardous materials such as asbestos. These improved substrates exhibit excellent caustic resistance and high permeability to electrolyte ion movement without promoting undesirable dendrite formation. Accordingly, it is a primary object of the present invention to provide a new and improved nonwoven battery separator substrate capable of exhibiting resistance to caustic degradation after prolonged high temperature exposure to concentrated alkaline conditions coupled with resistance to oxidation upon recharging, tolerance to deeper discharge cycling and reduced thickness and weight without sacrifice in strength.
Another object of the present invention is to provide a new and improved alkaline battery separator substrate that is of extremely low cost, is free of binder additives, and exhibits good electrical resistance yet provides excellent receptivity to porous battery separator impregnants or coatings, particularly coatings using an aqueous carrier.
Yet another object of the present invention is to provide an alkaline battery separator substrate that is comprised of low cost materials and is easy to manufacture on automated equipment and in large quantities thereby providing a substantial economic advantage in the battery made therefrom. Included in this object is the provision for an extremely thin battery separator substrate of light weight that facilitates the fabrication of a more compact battery design of enhanced operating performance.
Still another object of the present invention is the provision for a totally synthetic organic nonwoven substrate of the type described that exhibits high flexibility and the desired porosity yet high strength and structural integrity.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are achieved in accordance with the present invention by providing a nonwoven fibrous web material suited for use as a substrate for an alkaline battery separator comprised of a light weight, porous, heat bonded, synthetic organic sheet material having a basis weight of less than about 50 gsm and a thickness of less than about 200 microns, a major fibrous component of synthetic thermoplastic polyolefin fibers having a prefused microfibrillar structure to wood pulp and a minor fibrous component of a high tenacity polyamide fiber having a fiber length greater than about 6 mm. The heat bonding by partial fusion of the microfibrillar polyolefin is sufficient to impart to the sheet material a wet tensile strength of at least 400 g/in. width while retaining an air permeability of about 50 liters per minute and more. The substrate exhibits excellent receptivity to impregnation by pore-forming battery separator coatings.
A better understanding of the invention will be obtained from the following detailed description wherein the article of manufacture possesses the features, properties, and relation of elements described and exemplified herein.